The field of art to which this invention pertains is the operation of a continuous conversion process employing solid catalyst particles. More specifically, it relates to the thermal treatment of solid catalyst particles prior to the introduction of the catalyst particles into a catalytic reaction zone operating at an elevated temperature.
Catalytic processes for the conversion of hydrocarbons are well known and extensively used. Invariably, the catalyst used in these processes becomes deactivated for one or more reasons. In order to achieve a continuous operation for the conversion of hydrocarbons in a catalytic reaction zone, various processes have been developed wherein the catalyst particles are slowly moved through a catalytic reaction zone and are subsequently removed from the conversion zone when they become deactivated. The deactivated catalyst may then be regenerated, and introduced and reused in the catalytic reaction zone. In general, the catalytic reaction zone which employs moving bed technology is operated at severe conditions which normally include relatively high temperatures. With the combination of the processing plant operated at high temperatures and the introduction of fresh or regenerated catalyst introduced into the catalytic reaction zone, it becomes necessary to consider the thermal stress which is imposed upon the processing plant, including the catalytic reactor internals. Thermal stress causes problems such as equipment failures, for example, and thermal stress is greatly reduced or eliminated by the preheating of the catalyst. In the past, the new or regenerated catalyst which is to be subsequently introduced into the catalytic reaction zone has been introduced into a vessel which is relatively resistant to thermal stresses and contains little or no fragile internals which would be susceptible to thermal stress and damage. This type of a vessel utilized for the heating and temperature equilibration of the catalyst before its introduction into the catalytic reaction zone has traditionally been heated by a convenient gaseous stream which is preferably present in the process and which may be heated and introduced into the vessel utilized to preheat the catalyst. In processes for the catalytic reforming of naphtha and for the dehydrogenation of hydrocarbon compounds, elemental hydrogen is generated in the normal course of the reaction. This net hydrogen gas stream has been used in the past as a heat transfer agent which is heated and introduced into the catalyst preheat zone. Although such a hydrogen stream has a very high purity and generally contains at least about 90 mol percent hydrogen, it may frequently contain quantities of hydrocarbons and even trace quantities of olefinic or unsaturated hydrocarbons. During the separation of the products from the reaction zone, an effort is made to produce a stream consisting essentially of hydrocarbons and a hydrogen stream containing essentially no hydrocarbons. However, such a complete separation is difficult and expensive and, even if a high quality separation scheme is in place and if there is an unexpected operational upset, the purity of the net hydrogen gas stream may very often be compromised and thereby contain olefinic hydrocarbon compounds. It has been discovered that when a hydrogen-rich gaseous stream containing olefinic or unsaturated hydrocarbon compounds is heated for use in the subsequent heating of catalyst particles at high temperatures, the contacting of the olefinic hydrocarbons with the heat exchange surfaces produces fouling of the heat exchanger and reduced heat transfer coefficients which ultimately leads to the shutdown of the heat exchanger for cleaning.
We have unexpectedly discovered that if a hydrogen-rich gaseous stream containing olefinic hydrocarbons is contacted with a hydrogenation catalyst at relatively mild conditions before the gas stream is introduced into the heater, the problem of rapid fouling of the heat exchange surfaces is no longer observed.